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EXTERNAL CONTROL OF SEMICONDUCTOR
NANOSTRUCTURE LASERS
BY
NADER A. NADERI
B.S., Applied Physics, University of Mashhad, 2000 M.S., Electrical Engineering, University of New Mexico, 2007
DISSERTATION
Submitted in Partial Fulfillment of the Requirements for the Degree of
Doctor of Philosophy
Engineering
The University of New Mexico Albuquerque, New Mexico
July, 2011
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© 2011, Nader A. Naderi
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ACKNOWLEDGMENTS
I am foremost thankful to my advisor and dissertation committee chairman,
Professor Luke Lester for his support and expert guidance throughout the course of this
study. His patience and faith in my abilities has inspired me to find my career path and
learn a lot in the field of optoelectronics.
In addition, special thanks are due to my thesis committee members Professor
Christos Christodoulou, Professor Mani Hossein-Zadeh, and Professor Frederic Grillot
who provided me great help and insightful comments in completing this dissertation.
I wish to acknowledge my gratitude to Dr. Yan Li, Dr. Michael Pochet, Dr. Mark
Crowley, Nishant Patel and all of my other friends and colleagues at the Center for High
Technology Materials, who contributed their valuable assistance and helpful discussions
during completion of this research. My sincere thanks also go to Dr. Vassilios Kovanis,
and Dr. Nicholas Usechak at the Air Force Research Laboratory, Wright-Patterson AFB,
for their helpful advice and assistance during this investigation.
I am truly grateful to my beloved wife Shadi, for her endless support, patience,
and understanding through these years. Lastly, I wish to dedicate this achievement to my
family for all the love, support and encouragement.
This investigation was supported by the United States Air Force Research
Laboratory, and founded by the Air Force Office of Scientific Research at Center for
High Technology Materials, University of New Mexico.
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EXTERNAL CONTROL OF SEMICONDUCTOR
NANOSTRUCTURE LASERS
BY
NADER A. NADERI
ABSTRACT OF DISSERTATION
Submitted in Partial Fulfillment of the Requirements for the Degree of
Doctor of Philosophy
Engineering
The University of New Mexico Albuquerque, New Mexico
July, 2011
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External Control of Semiconductor
Nanostructure Lasers
by
Nader A. Naderi
B.S., Applied Physics, University of Mashhad, 2000
M.S., Electrical Engineering, University of New Mexico, 2007
Ph.D., Engineering, University of New Mexico, 2011
ABSTRACT
Novel semiconductor nanostructure laser diodes such as Quantum-Dot (QD) and
Quantum-Dash (QDash) are key optoelectronic components for many applications such
as ultra fast optical communication. This is mainly due to their unique carrier dynamics
compared to conventional quantum-well (QW) lasers that enables their potential for high
differential gain and modified linewidth enhancement factor. However there are known
intrinsic limitations associated with the semiconductor laser dynamics that can hinder its
ultimate performance including the mode stability, linewidth, and direct modulation
capabilities. One possible method to overcome these limitations is through external
control techniques. The electrical and/or optical external perturbations can be
implemented to improve the parameters associated with the laser’s dynamics, such as
threshold gain, damping, spectral linewidth, and mode selectivity. This work studied the
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impact of external control techniques through optical injection-locking, optical feedback
and asymmetric bias control on the overall performance of the nanostructure lasers in
order to understand the associated intrinsic limitations and to develop strategies for
controlling the underlying dynamics to improve the laser performance.
In this document, the impact of external control through optical injection-locking on
the modulation characteristics of QD and QDash lasers are investigated. Using the
conventional rate equations describing an injection-locked system, a novel modulation
response function is derived which implicitly incorporates non-linear gain through the
free-running relaxation oscillation frequency and damping rate of the slave laser. It is
shown that the model presented can be used to accurately model and extract the
characteristic parameters of the coupled system directly from measured microwave
experimental data. The significance of this modeling approach is that it allows all the
external control parameters to be extracted in the frequency domain where they can be
easily compared in order to further aid in optimizing the modulation performance of the
system. Using the simulation results, the impact of intrinsic slave parameters, including
the free-running relaxation oscillation, linewidth enhancement factor, and damping rate,
on the injection-locked modulation transfer function are investigated. The impact of ultra-
strong optical-injection on the slave linewidth enhancement factor found in QD and
QDash lasers are analyzed using theoretical predictions and verified with experimental
observations. The experimental findings presented in this dissertation show that the free-
running linewidth enhancement factor in nanostructure lasers can be manipulated due to
the significant threshold gain shift under strong optical injection. This novel finding
along with the enhanced bandwidth advantages offered in the direct modulation of
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injection-locked nanostructure lasers promises a path to realizing a new generation of
compact, chirp-free, and ultrafast (>100 Gb/s) optical sources for data transmission.
Using an external optical feedback stabilization method and/or asymmetric bias
control, a dual-wavelength emission mechanism is realized in a two-section QD
distributed feedback (DFB) laser. It is shown that under asymmetric bias conditions, the
powers between the ground-state and excited-state modes of the two-section device can
be equalized, which is mainly attributed to the unique carrier dynamics of the QD gain
medium. It is also found that the combination of significant inhomogeneous broadening
and excited-state coupled mode operation allows the manipulation of the QD states
through external optical stabilization. The technical design and external control
approaches described in this study along with current on-chip photomixing capabilities
have potential in engineering a compact and low-cost THz source for future applications.
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TABLE OF CONTENTS
ABSTRACT ...............................................................................................................vi
LIST OF FIGURES ..................................................................................................xiv
LIST OF TABLES ..................................................................................................xxii
LIST OF PUBLICATIONS ASSOCIATED WITH THIS STUDY ......................xxiii
Chapter 1 .................................................................................................................1
External Control Techniques in Semiconductor Nanostructure Lasers
1.1 Introduction .......................................................................................................1
1.2 Motivation for this Study ..................................................................................4
1.2.1 Optical Injection-Locking .....................................................................7
Overview of the Injection-Locking Technique .............................................7
A Brief History of Optical Injection-Locking ...........................................9
Applications of Optically Injection-Locked Laser ......................................12
1.2.2 External Control through Optical Feedback .......................................14
Overview of External Optical feedback .......................................................14
Regimes of Optical Feedback .......................................................................16
Advantages of Controlled Optical Feedback ...............................................17
Applications of Semiconductor Lasers with External Optical Feedback
...........................................................................................................19
1.2.3 External Control through Monolithic Multi-section Design
..............................................................................................................19
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Optical Gain-Lever .......................................................................................20
Multi-section DFB/DBR Lasers ..................................................................22
1.3 Organization of Dissertation ...........................................................................23
1.4 Chapter 1 References ......................................................................................25
Chapter 2 ...............................................................................................................42
Modeling the Injection-Locked Characteristics of Nanostructure
Semiconductor Lasers
2.1 Introduction .....................................................................................................42
2.2 Injection-Locking Theoretical Model .............................................................44
2.2.1 Rate Equations .....................................................................................44
2.2.2 Steady-State Solutions .........................................................................46
2.2.3 Small-Signal Analysis (Dynamic Solutions) .......................................49
2.2.4 Modulation Response Function ...........................................................50
2.2.5 Key Frequency Detuning Cases ...........................................................54
2.2.6 Identifying the Known Free-Running Parameters ...............................61
2.3 Free-Running Characterization – Experimental Setup ...................................62
2.4 Injection-Locking Characterization – Experimental Setup .............................65
2.5 Modeling the Injection-Locking Characteristics of QDash Laser ..................67
2.5.1 Description of QDash Fabry-Perot Laser ..........................................67
2.5.2 Determining the QDash Laser Free-Running Parameters and the
Nonlinear Effects ...............................................................................70
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2.5.3 Modulation Response of the Injection-Locked QDash FP Device:
Experimental Findings ......................................................................73
2.5.4 Fitting Parameter Constraints ............................................................74
2.5.5 Curve-Fitting Results ........................................................................78
2.5.6 Model-Based Analysis ......................................................................81
2.5.7 Temperature Effects on the Injection-Locked QDash Laser
Modulation Response ........................................................................84
2.6 Modeling the Injection-Locked Characteristics of QD FP Laser ...................89
2.6.1 Description of QD Fabry-Perot Laser ...............................................89
2.6.2 Determining the QDash Laser Free-Running Parameters and the
Nonlinear Effects ...............................................................................92
2.6.3 Modulation Response of the Injection-Locked QD FP Device:
Experimental Findings ......................................................................98
2.6.4 Extracting the Operating Parameters of the QD FP Device ............109
2.6.5 Curve-Fitting Results ......................................................................111
2.7 Chapter 2 Summary ......................................................................................116
2.8 Chapter 2 References ....................................................................................118
Chapter 3 .............................................................................................................123
Manipulation of the Linewidth Enhancement Factor in QDash Nanostructure
Laser under Strong Optical Injection
3.1 Introduction ...................................................................................................123
3.1.1 α-factor in QD and QDash Nanostructure Lasers ...........................124
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3.1.2 Controlling the α-factor through Optical Injection-Locking ..........125
3.1.3 High-Level Objectives of the α-factor Investigation ......................126
3.2 Characterization of the Material α-factor in a QDash Laser ........................128
3.2.1 ASE/Hakki-Paoli Technique for Determining the Material α-factor
..........................................................................................................129
3.3 Direct Measurement of the Above Threshold α-factor in QDash Laser under
Optical Injection ............................................................................................134
3.3.1 Description of the FM/AM Modulation Technique ........................134
3.3.2 FM/AM Measurement - Experimental setup ...................................138
3.3.3 FM/AM Measurement - Experimental Results ...............................141
3.4 Extracting the α-factor and Threshold Gain Shift using the Zero-Detuning
Modulation Response Data ...........................................................................142
3.5 Chapter 3 References ....................................................................................153
Chapter 4 .............................................................................................................158
Two-Color Multi-Section Quantum Dot Distributed Feedback Laser Diode
4.1 Introduction ...................................................................................................158
4.2 Motivation for the Two-Color Multi-Section Laser .....................................159
4.3 DFB Device Structure, Fabrication and Characterization ............................161
4.3.1 InAs/InGaAs QD Structure and Fabrication ...................................162
4.3.2 LLC-DFB Device Light-Current Characteristics ............................164
4.3.3 Evaluation of the Coupling Coefficient in LLC-DFB Laser ...........166
4.4 Generation of Two-Color Emission through Asymmetric Pumping ............171
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4.5 Generation of Two-Color Emission through Applying External Optical
Feedback .......................................................................................................174
4.6 Chapter 4 References ....................................................................................177
Chapter 5 .............................................................................................................183
Summary and Conclusion
5.1 Summary and Conclusion .............................................................................183
5.2 Proposed Future research ..............................................................................188
5.3 Chapter 5 References ....................................................................................191
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LIST OF FIGURES
Figure (1.1) Schematic diagram of an optically injection-locked laser system ........….8
Figure (1.2) Optical spectra of an injection-locked Fabry-Perot Laser .........................8
Figure (1.3) Measured modulation responses of an injection-locked FP laser, for
various detuning conditions, indicating free-running, broadband and
narrowband responses .............................................................................11
Figure (1.4) Regimes of optical feedback for a DFB laser indicating the feedback
power ratio at which the transition between regimes occur as a function
of external round-trip time .......................................................................16
Figure (1.5) Schematic diagram of a two-section gain-lever semiconductor laser with
the evolution of gain versus carrier density .............................................21
Figure (2.1a) Simulation of the modulation response function under zero, extreme
positive and extreme negative frequency detuning at constant injection
strength ....................................................................................................55
Figure (2.1b) Simulation of modulation response function under positive frequency
detuning edge, for various values of injection strength ...........................55
Figure (2.1c) Simulation of modulation response function under zero detuning, for
various values of injection strength ………...….........……………....….58
Figure(2.2a) Schematic of the free-running device characterization experimental setup
...................................................................................................................63
Figure (2.2b) Picture of the high-speed configuration showing the test laser, ground
pad and schematic of 40GHz signal-ground Pico-probe ........................63
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Figure(2.3) Block diagram of the injection-locking experimental setup ....................65
Figure(2.4a) Epitaxial layer structure of the InAs QDash laser ..................................68
Figure (2.4b) AFM image of the InAs QDash laser material ........................................68
Figure (2.5) Wide-span spectra of the QDash FP laser at bias current of 60 mA ......69
Figure (2.6a) Measured free-running modulation response of the QDash FP device
biased at 60 mA and the least-squares fitting results ..............................71
Figure (2.6b) Free-running damping factor as a function of the square of the relaxation
oscillation frequency ........................................................................ ........71
Figure (2.7) Square of the relaxation oscillation frequency versus the free-running
output power ............................................................................................72
Figure (2.8) Modulation responses of the free-running and the injection-locked QDash
laser under zero, positive and negative frequency detuning conditions
...................................................................................................................74
Figure (2.9) Variation of linewidth enhancement factor as a function of applied bias
current in the QDash FP laser ................................................................77
Figure (2.10a) Simulations of the modulation response function for various values of the
linewidth enhancement factor ..................................................................82
Figure (2.10b) Experimental observation of the varied sag as the linewidth enhancement
factor is increased ....................................................................................82
Figure (2.11) Light-current characteristics of the QDash FP device for various
temperatures ............................................................................................85
Figure (2.12) Variation in the QDash free-running relaxation frequency and damping
rate vs. temperature .................................................................................86
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Figure (2.13) Variation in the QDash FP linewidth enhancement factor as a function of
temperature ..............................................................................................86
Figure (2.14) Improvement in the injection-locked modulation responses at positive
frequency detuning vs. temperature variation .........................................87
Figure (2.15) a) Epitaxial layer structure of the InAs/GaAs QD slave laser, b) Light-
current characteristics and emission spectra of the QD slave laser under
investigation .............................................................................................91
Figure (2.16) Normalized modulation response of the free-running slave laser for
various pump currents .............................................................................93
Figure (2.17) Variation of the slave free-running relaxation oscillation and damping
rate as a function of bias current .............................................................93
Figure (2.18) QD free-running damping factor as a function of the relaxation
oscillation frequency squared ..................................................................95
Figure (2.19) QD free-running relaxation oscillation frequency squared as a function of
total output power ....................................................................................95
Figure (2.20) Variation of the slave linewidth enhancement factor as a function of
applied bias current for a 1310 nm QD FP laser ....................................96
Figure (2.21) Free-running and injection-locked spectra of the QD FP laser under zero
detuning condition at 1312.33 nm, indicating a >30dB SMSR between the
locked mode and side FP modes ..............................................................99
Figure (2.22) Normalized modulation responses under positive (top) and negative
(bottom) frequency detuning conditions for Pext-inj=7dB, indicating 2.6X
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and 4.6X improvement in 3-dB bandwidth compared to the free-running
case ........................................................................................................100
Figure (2.23) Normalized modulation responses under positive (top) and negative
(bottom) frequency detuning conditions for Pext-inj=12dB, indicating 4.4X
and 6X improvement in 3-dB bandwidth compared to the free-running
case ........................................................................................................101
Figure (2.24) Normalized modulation responses under positive (top) and negative
(bottom) frequency detuning conditions for Pext-inj=15dB, indicating 8.1X
and 4.7X improvement in 3-dB bandwidth, respectively, compared to the
free-running case ...................................................................................102
Figure (2.25) Modulation responses under positive and negative frequency detuning
conditions for Pext-inj=7dB, indicating the modulation efficiency decreases
by 27dB and 38dB, respectively, compared to the free-running case
.................................................................................................................104
Figure (2.26) Modulation responses under positive and negative frequency detuning
conditions for Pext-inj=12dB, indicating the modulation efficiency
decreases by 26dB and 35dB, respectively, compared to the free-running
case ........................................................................................................105
Figure (2.27) Modulation responses under positive (top) and negative (bottom)
frequency detuning conditions for Pext-inj=15dB, indicating the modulation
efficiency decreases by 3dB, and slight increase by 5dB, respectively,
compare to the free-running case ..........................................................106
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Figure (2.28) Normalized modulation responses of the free-running and the injection-
locked QD laser under various frequency detuning conditions and
injection strengths ..................................................................................108
Figure (2.29) Least-squares fit results of the normalized modulation response of the
free-running and the injection-locked QD laser for Pext= 7dB and ∆f = -
6.03 GHz, Pext= 12dB and ∆f = -5.97 GHz, Pext= 15dB and ∆f = 9.1 GHz
.................................................................................................................112
Figure (3.1) Net modal gain as a function of wavelength calculated from the peak-to-
valley ratios in ASE spectra ranging from 1470.6 A/cm2 to 2700 A/cm2
.................................................................................................................130
Figure (3.2) Peak wavelength as a function of duty cycle at J=1941.2 A/cm2.The pulse
width was kept constant at 6 µs for all cases .........................................132
Figure (3.3) Variation of the material α-factor as a function of wavelength taken near
threshold at current density value of 2647.1 A/cm2 ...............................132
Figure (3.4) Sample contour diagram used to extract m and β from graphical solution
.................................................................................................................137
Figure (3.5) Block diagram of the experimental setup used to characterize the
modulation response and α-factor of the injection-locked FP QDash
using the FM/AM modulation technique ................................................138
Figure (3.6) Free-Running and zero detuning injection-locked spectra of the QDash
FP slave laser at different wavelengths .................................................139
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Figure (3.7) Sample injection-locked QDash FM/AM response as a function of
modulation frequency curve-fitted to (3.8) to characterize the α-factor
.................................................................................................................140
Figure (3.8) Measured α-factor as a function of Rext at zero-detuning cases using
FM/AM modulation technique ...............................................................141
Figure (3.9) Free-running and injection-locked modulation responses at zero detuning
as a function of Rext. The zero-detuning response data corresponds to the
optical injection locking near the gain peak wavelength at 1565.2 nm. The
experimental response data are curve-fitted using the simplified
modulation response function ................................................................143
Figure (3.10) Comparison between measured and extracted α-factor and extracted
threshold gain shift at zero-detuning cases as a function of Rext. The
measured and extracted values correspond to the injection-locking case
near the gain peak wavelength at 1565.2 nm ........................................145
Figure (3.11a) Free-running and injection-locked modulation responses at zero detuning
as a function of Rext for the mode at 1579.9 nm. The experimental
response data are curve-fitted using the simplified modulation response
function ..................................................................................................146
Figure (3.11b) Comparison between measured and extracted α-factor and extracted
threshold gain shift at zero-detuning cases as a function of Rext. The
measured and extracted values correspond to the injection-locking case
at 1579.9 nm ...........................................................................................146
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Figure (3.12a) Free-running and injection-locked modulation responses at zero detuning
as a function of Rext for the mode at 1550.1 nm. The experimental
response data are curve-fitted using the simplified modulation response
function ..................................................................................................147
Figure (3.12b) Comparison between measured and extracted α-factor and extracted
threshold gain shift at zero-detuning cases as a function of Rext. The
measured and extracted values correspond to the injection-locking case
at 1550.1 nm ...........................................................................................147
Figure (3.13a) Free-running and injection-locked modulation responses at zero detuning
as a function of Rext for the mode at 1534.6 nm. The experimental
response data are curve-fitted using the simplified modulation response
function ..................................................................................................148
Figure (3.13b) Comparison between measured and extracted α-factor and extracted
threshold gain shift at zero-detuning cases as a function of Rext. The
measured and extracted values correspond to the injection-locking case
at 1534.6 nm ...........................................................................................148
Figure (3.14) Calculated threshold gain shift of zero-detuning cases as a function of Rext
at 1580 nm, 1565.2 nm, 1550nm, and 1535nm ......................................150
Figure (4.1a) (a) Oblique schematic view of the epitaxial layers and two-section cavity
structure of the InAs QD LLC-DFB laser. (b) Oblique SEM image of the
100 nm wide chromium grating lines adjacent to the ridge waveguide
processed by electron-beam lithography and metal evaporation ..........163
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Figure (4.1b) The flow chart of the processing procedure for a two-section LLC DFB
laser ........................................................................................................163
Figure (4.2) (a) Room temperature Light-Current characteristics of the two-section
QD DFB laser. (b) Broad optical spectrum of the two-section DFB laser
at 110 mA indicating the existence of the ES and GS peaks under uniform
pumping condition .................................................................................165
Figure (4.3) Sub-threshold spectra of the two-section LLC DFB laser biased uniformly
at 60 mA .................................................................................................168
Figure (4.4) Sub-threshold spectra of the two-section LLC DFB laser biased uniformly
at 70 mA .................................................................................................168
Figure (4.5) Extracted values of the gain coupling coefficient for LLC DFB laser as a
function of uniform bias current ranges from 60 mA -70mA .................170
Figure (4.6) Extracted values of the gain coupling coefficient for LLC DFB laser as a
function of uniform bias current ranges from 60 mA -70mA .................170
Figure (4.7) Wide-span spectra of the QD DFB laser under uniform and asymmetric
bias conditions. Under asymmetric pumping, the SMSR for the GS
emission is 14 dB ...................................................................................172
Figure (4.8) Broad optical spectra at uniform bias of 110 mA under free-running (no
feedback) and external optical feedback level ranges from -50 dB to -25
dB ...........................................................................................................175
Figure (4.9) Schematic diagram of the experimental optical feedback setup ............175
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LIST OF TABLES
Table (2.1) Fixed fitting parameters and summary of constraints used in the least-
squares-fitting under a bias current of 60 mA ....................................….78
Table (2.2) Extracted injection-locking operating parameters from least-squares-
fitting of experimental data with response model in (2.31) .....................80
Table (2.3) Extracted operating parameters for the injection-locked conditions shown
in Figure (2.29) ......................................................................................112
Table (2.4) Calculated Parametric C and Z terms at ∆f =9.1 GHz and ∆f =0 GHz for
Pext = 15dB .............................................................................................116
Table (3.1) Extracted injection-locking operating parameters at 1579.9 nm from least-
squares-fitting of experimental data .......................................................149
Table (3.2) Extracted injection-locking operating parameters at 1550.1 nm from least-
squares-fitting of experimental data .......................................................149
Table (3.3) Extracted injection-locking operating parameters at 1534.6 nm from least-
squares-fitting of experimental data .......................................................149
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LIST OF PUBLICATIONS ASSOCIATED WITH THIS STUDY
Refereed Journal Articles and Book Chapter
N. A. Naderi, M. Pochet, F. Grillot, N. Terry, V. Kovanis, L. F. Lester, “Modeling the
Injection-Locked Behavior of a Quantum-Dash Semiconductor Laser,” IEEE
Journal of Selected Topics in Quantum Electronics, Vol. 15, No. 3, pp. 563-571,
2009.
N. A. Naderi, F. Grillot, K. Yang, J. B. Wright, A. Gin, and L. F. Lester, “Two-Color
Multi-Section Quantum Dot Distributed Feedback Laser,” Optics Express, Vol. 18,
No. 26, pp. 27028–27035, 2010.
M. Pochet, N. A. Naderi, V. Kovanis, and L. F. Lester, “Modeling the Dynamic
Response of an Optically-Injected Nanostructure Diode Laser,” IEEE Journal of
Quantum Electronics, Vol. 47, No. 6, pp. 827-833, 2011.
M. Pochet, N. A. Naderi, Y. Li, V. Kovanis, L. F. Lester, “Tunable Photonic Oscillators
Using Optically Injected Quantum-Dash Diode Lasers,” IEEE Photonics
Technology Letters, Vol. 22, No. 11, pp. 763-765, 2010.
F. Grillot, N. A. Naderi, M. Pochet, C.-Y. Lin, and L. F. Lester, “The Critical Feedback
Level in Nanostructure-Based Semiconductor Lasers,” Book chapter in
Semiconductor Technologies, Published by In-Tech, ISBN 978-953-307-080-3,
April 2010.
M. Pochet, N. A. Naderi, N. Terry, V. Kovanis, L. F. Lester, “Dynamic Behavior of an
Injection-Locked Quantum-Dash Fabry-Perot Laser at Zero Detuning,” Optics
Express, Vol. 17, No. 23, pp. 20623-20630, 2009.
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F. Grillot, N. A. Naderi, M. Pochet, C.-Y. Lin, P. Besnard, and L. F. Lester, “Tuning of
the Critical Feedback Level in 1.55-µm Quantum-Dash Semiconductor Laser
Diodes,” IET Optoelectronics, Vol. 3, No. 6, pp. 242-247, 2009.
N. B. Terry, N. A. Naderi, M. C. Pochet, A. J. Moscho, L. F. Lester and V. Kovanis,
“Bandwidth Enhancement of Injection-Locked 1.3-µm Quantum-Dot DFB Laser”,
Electronic Letters, Vol. 44, No. 15, pp. 904-905, 2008.
F. Grillot, N. A. Naderi, M. Pochet, C.-Y. Lin, and L. F. Lester, “Variation of the
Feedback Sensitivity in a 1.55-µm InAs/InP Quantum-Dash Fabry-Perot
Semiconductor Laser,” Applied Physics Letters, Vol. 93, pp. 191108-1-3, 2008.
Conference Proceedings
N. A. Naderi, M. C. Pochet, F. Grillot, A. Shirkhorshidian, V. Kovanis, L. F. Lester,
“Manipulation of the Linewidth Enhancement Factor in an Injection-Locked
Quantum-Dash Fabry-Perot Laser at 1550nm,” In proceedings of the 2010
IEEE/Photonics Society conference in Denver, CO.
N. A. Naderi, M. Pochet, V. Kovanis, L. F. Lester, “Bandwidth Enhancement in an
Injection-Locked Quantum Dot Laser Operating at 1.31-µm,” In proceedings of the
2010 Photonic West Conference in San Francisco, CA.
N. A. Naderi, M. Pochet, F. Grillot, Y. Li, and L. F. Lester, “Temperature Effects on the
Modulation Response of an Injection-Locked InAs/InP Quantum-Dash Laser,” In
proceedings of the 2009 IEEE/IPRM Conference in Newport Beach, CA.
N. A. Naderi, M. Pochet, F. Grillot, N. Terry, V. Kovanis, L. F. Lester, “Extraction of
Physical Parameters from an Injection-Locked Quantum-Dash Fabry-Perot
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Laser,” In proceedings of the 2008 IEEE/LEOS Society Meeting in Newport
Beach, CA.
M. Pochet, N. A. Naderi, V. Kovanis, L. F. Lester, “Optical-Injection of Quantum-
Dashes Semiconductor Lasers at 1550nm for Tunable Photonic Oscillators,” In
proceedings of the 2011 Photonic West Conference in San Francisco, CA.
F. Grillot, N. A. Naderi, C.-Y. Lin, K. Yang, A. Gin, A. Shirkhorshidian, and L. F.
Lester, “Two-Color Quantum-Dot DFB Laser for Terahertz Applications,” In
proceedings of the 2010 IEEE/Photonics Society conference in Denver, CO.
M. Pochet, N. A. Naderi, N. Terry, V. Kovanis, L. F. Lester, “Linewidth Enhancement
factor and Dynamical Response of an Injection-Locked Quantum-Dot Fabry-Perot
Laser at 1310nm,” In proceedings of the 2010 Photonic West Conference in San
Francisco, CA.
M. Pochet, N. A. Naderi, V. Kovanis, L. F. Lester, “Optically Injected Quantum Dash
Lasers at 1550nm Employed as Highly Tunable Photonic Oscillators,” In
proceedings of the 2010 CLEO/QELS Conference in San José, CA.
F. Grillot, N. A. Naderi, M. Pochet, C.-Y. Lin, and L. F. Lester, “Influence of the
Linewidth Enhancement Factor on the Critical Feedback Level in a Quantum Dash
Laser,” In proceedings of the 2009 CLEO/IQEC Conference in Baltimore, MD.
F. Grillot, N. A. Naderi, M. C. Pochet, C.-Y. Lin, P. Besnard, and L. F. Lester, “Tuning
of the Critical Feedback Level in 1.5-µm Quantum Dot Semiconductor Lasers,” In
proceedings of the 2009 SIOE Conference in Cardiff, United Kingdom.
F. Grillot, N. A. Naderi, M. C. Pochet, C.-Y. Lin, and L. F. Lester, “Systematic
Investigation of the Alpha Parameter Influence on the Critical Feedback Level in
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QD Lasers,” In proceedings of the 2009 Photonic West Conference in San José,
CA.
M. Pochet, N. A. Naderi, F. Grillot, N. Terry, V. Kovanis, L. F. Lester, “Methods for
Improved 3dB bandwidth in an Injection-Locked QDash Fabry Perot Laser at
1550nm,” In proceedings of the 2009 CLEO/IQEC Conference in Baltimore, MD.
M. Pochet, N. A. Naderi, F. Grillot, N. Terry, V. Kovanis, L. F. Lester, “Modulation
Response of an Injection Locked Quantum-Dash Fabry Perot Laser at 1550nm,” In
proceedings of the 2009 Photonic West Conference in San José, CA.
V. Kovanis, N. G. Usechak, N. A. Naderi, M. Pochet, and L. F. Lester, “Ultrafast Diode
Lasers Via Strong Optical Injection,” In proceedings of the 2009 ETOPIM
Conference in Crete, Greece.
F. Grillot, N. A. Naderi, M. C. Pochet, C.-Y. Lin and L. F. Lester, “Variation of the
Critical Feedback Level in a 1550nm Quantum-Dash Fabry-Perot Semiconductor
Laser,” In proceedings of the 2008 IEEE/LEOS Society Meeting in Newport
Beach, CA.
L. F. Lester, N. B. Terry, A. J. Moscho, M. L. Fanto, N. A. Naderi, Y. Li, and V.
Kovanis, “Giant nonlinear gain coefficient of an InAs/AlGaInAs quantum dot
laser,” In proceedings of the 2008 SPIE Photonic West Conference in San José,
CA.
N. Terry, N. A. Naderi, M. C. Pochet, L. F. Lester, “3-dB Bandwidth Enhancement via
Strong Optical Injection-Locking of a Quantum Dot DFB at 1310 nm,” In
proceedings of the 2008 IEEE/LEOS Society Meeting in Newport Beach, CA.
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Y. Li, N. A. Naderi, V. Kovanis, L. F. Lester, “Modulation Response of an Injection-
Locked 1550 nm Quantum Dash Semiconductor Laser,” In proceedings of the 2007
IEEE/LEOS Society Meeting in Lake Buena Vista, FL.
Other Publications
Y. Li, N. A. Naderi, V. Kovanis, L. F. Lester, “Enhancing the 3-dB Bandwidth via the
Gain-Lever Effect in Quantum-Dot Lasers,” IEEE Photonics Journal, Vol. 2, No. 3,
pp. 321-329, 2010.
C.-Y. Lin, F. Grillot, N. A. Naderi, Y. Li, and L. F. Lester, “RF Linewidth Reduction in
a Quantum Dot Passively Mode-Locked Laser Subject to External Optical
Feedback,” Applied Physics Letters, Vol. 96, No. 5, pp. 051118, 2010.
J. H. Kim, C-Y. Lin, N. A. Naderi, Y.-C. Xin, L. F. Lester and C. G. Christodoulou,
“Pattern Estimation of a Microstrip Antenna Integrated With a Quantum Dot
Mode-Locked Laser,” IEEE Antennas and Wireless Propagation Letters, Vol. 9, pp.
954-957, 2010.
F. Grillot, C.-Y. Lin, N. A. Naderi, M. Pochet, L. F. Lester, “Optical Feedback
Instabilities in a Monolithic InAs/GaAs Quantum Dot Passively Mode-Locked
Laser,” Applied Physics Letters, Vol. 94, pp. 153503-1-3, 2009.
J. H. Kim, C. G. Christodoulu, Z. Ku, C.-Y. Lin, Y.-C. Xin, N. A. Naderi, and L. F.
Lester, “Hybrid Integration of a Bowtie Slot Antenna and a Quantum Dot Mode-
Locked Laser,” IEEE Antennas and Wireless Propagation Letters, Vol. 8, pp. 1337-
1340, 2009.
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N. G. Usechak, N. A. Naderi, M. Grupen, Y. Li, L. F. Lester, and V. Kovanis, “Cavity-
Enhanced Modulation in Gain-Lever Semiconductor Lasers,” In proceedings of the
2010 IEEE/Photonics Society conference in Denver, CO.
C.-Y. Lin, F. Grillot, N. A. Naderi, Y. Li, L. F. Lester, “Ultra-low RF Linewidth in a
Quantum Dot Mode-Locked Laser Under External Optical Feedback
Stabilization,” In proceedings of the 2010 CLEO/QELS Conference in San José,
CA.
J. H. Kim, C.-Y. Lin, Y. Li, N. A. Naderi, C. G. Christodoulou, and L. F. Lester, “Beam
steering of a linearly tapered slot antenna array integrated with quantum dot
mode-locked lasers,” In proceedings of the 2010 IEEE/Photonics Society
conference in Denver, CO.
C.-Y. Lin, N. A. Naderi, F. L. Chiragh, J. Kim, C. G. Christodoulou and L. F. Lester,
“31% DC to RF Differential Efficiency Using Monolithic Quantum Dot Passively
Mode-Locked Lasers,” In proceedings of the 2009 CLEO/IQEC Conference in
Baltimore, MD.
J. H. Kim, C. G. Christodoulou, Z. Ku, C.-Y Lin, N. A. Naderi, L. F. Lester, J. P. Kim,
“A Bowtie Slot Antenna coupled to a Quantum-Dot Mode Locked Laser,” In
proceedings of the 2009 IEEE International Symposium on Antennas and
Propagation and USNC/URSI (U.S. National Committee of the International Union
of Radio Science) in Charleston, SC.
C.-Y. Lin, Y.-C. Xin, N. A. Naderi, F. L. Chiragh and L. F. Lester, “Monolithic 1.58-
micron InAs/InP quantum dash passively mode-locked lasers,” In proceedings of
the 2009 Photonic West Conference in San José, CA.
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F. Grillot, C.-Y. Lin, N. A. Naderi, M. Pochet, and L. F. Lester, “Effects of Optical
Feedback in InAs/GaAs Monolithic Quantum Dot Passively Mode-Locked Lasers,”
In proceedings of the 2009 CLEO/IQEC Conference in Baltimore, MD.
J. H. Kim, C. G. Christodoulou, L. F. Lester, Y.-C. Xin, N. A. Naderi, Z. Ku, “Quantum-
Dot Laser Coupled Bowtie Antenna,” In proceedings of the 2008 IEEE
International Symposium on Antennas and Propagation and USNC/URSI in San
Diego, CA.
Y. Li, N. A. Naderi, Y.-C. Xin, C. Dziak, and L. F. Lester, “Multi-section gain-lever
quantum dot lasers,” In proceedings of the 2007 Photonic West Conference in San
José, CA.
Y. Li, N. A. Naderi, Y.-C. Xin, V. Kovanis, L. F. Lester, “Two-Section Quantum Dot
Lasers with 20-dB Modulation Efficiency Improvement,” In proceedings of the
2007 CLEO/QELS Conference in Baltimore, MD.
N. A. Naderi, Y. Li, C. Dziak, Y. C. Xin, V. Kovanis, L. F. Lester, “Quantum Dot Gain-
Lever Laser Diode,” In proceedings of the 2006 IEEE/LEOS Society Meeting in
Montreal, Canada.
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1
Chapter 1
External Control Techniques in Semiconductor Nanostructure Lasers
1.1 Introduction
Semiconductor coherent light-emitting diodes are one of the key optoelectronic
components that have been widely used in many fields such as ultrafast optical
communication systems, spectroscopy, remote sensing, and optical data storage. The
intrinsic dynamics of semiconductor lasers are unique depending on their structural
design and the make-up of their material system. Over the past 50 years there have been
many efforts devoted to improving semiconductor laser performance by improving the
internal laser properties through the invention of novel semiconductor material systems
and designing better cavity structures or by implementing external control techniques to
enhance the laser characteristics. With the developments in crystal growth technology
and the invention of novel semiconductor materials, the internal performance of
semiconductor lasers has been significantly improved over the past few decades. The
following is an up-to-date and brief summary of the advancement in growth of novel
semiconductor structures used in laser devices.
A definite breakthrough in the field of semiconductor lasers was the invention of
double heterostructure (DH) lasers in 1963 [1] in which both carrier and optical mode
confinement [2]-[6] improvements resulted in reduction of the threshold current density
and also enabled continuous wave (CW) operation at room temperature[7], [8]. Further
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2
developments in semiconductor material systems were achieved by advancements in
crystal growth techniques and realizing the concept of quantum size effects which
resulted in the first demonstration of a quantum well (QW) laser structure in 1977 [9],
[10]. In the QW material system, the carriers are confined within quantized energy levels
due to the reduction in the physical space volume in one dimension which helps to reduce
the threshold current density and allows for control of the lasing wavelength by changing
the quantum well thickness [11]-[12]. The advantages of one dimensional confinement
in QWs motivated more efforts to study the higher orders of carrier confinement, which
lead to the development of new nanostructure materials systems such as quantum dot
(QD) and quantum dash (QDash) [13]-[15]. The interest in QD and QDash arises from
their unique carrier dynamics resulting form three-dimensional (3-D) carrier confinement.
This confinement in all directions leads to discrete quantized energy levels that can be
controlled by changing the size and shape of the nanostructures. Predicted by Arakawa et.
al, semiconductor laser active regions made from 3-D confinement systems should
exhibit better internal performance compared to QW active region lasers [16], [17]. The
potential internal improvements in QD materials have been experimentally verified on
actual laser devices, which include low transparency current density [18], less
temperature dependence of threshold current density [19], increased gain and differential
gain [17], and a reduced linewidth enhancement factor [20]. Despite all the improvements
made in 3-D confinement material systems, there are still several limitations associated
with their carrier dynamics that can hinder the ultimate laser performance including the
mode stability, linewidth, and direct modulation capabilities.
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3
A clear example of these limitations can be seen in the direct modulation of
semiconductor lasers. Directly-modulated semiconductor lasers have become one of the
most efficient candidates for high-speed communication in microwave frequencies
because of their compactness and relatively low fabrication cost. Compared to external
modulation techniques, directly-modulated lasers commonly used in optical fiber link,
have simpler driver electronics and lower power consumption. Direct modulation
involves changing the current input around the bias level above threshold. It is principally
a simpler method and is easier to implement rather than external modulation, but the
output light produced depends on the laser’s complex internal dynamics. For instance,
compared to QW lasers, higher gain and differential gain in nanostructure QD lasers
would typically be expected to contribute to a larger modulation bandwidth [21].
Furthermore QD nanostructure lasers are known to exhibit near zero linewidth
enhancement factors at threshold which theoretically predicts a chirp-free direct
modulation performance [20]. In reality, strong gain saturation with carrier density in QD
lasers as result of inhomogeneous broadening prevents the laser from reaching its
ultimate high-speed performance. This is because the strong gain saturation in QDs
causes both the damping effect and the linewidth enhancement factor to significantly
increase with carrier density [22].
In general, high frequency chirp, small relaxation oscillation frequency, limited
output power, excessive noise, and high distortion limit the high-speed performance of
semiconductor lasers to transmissions at bit rates below 40 Gb/s. In order to improve the
modulation characteristics, such as obtaining higher modulation bandwidth or minimal
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4
frequency chirp, we need to be able to control and manipulate some of the intrinsic laser
parameters such as optical gain and/or the linewidth enhancement factor.
1.2 Motivation for this Study
One possible method for improving the overall performance of semiconductor
lasers is through various external control techniques. The original interest in using
external control techniques in semiconductor lasers is motivated from a desire to
understand the associated limitations and instabilities and to develop strategies for
controlling the underlying dynamics to improve the laser performance. Using these
techniques, external electrical and/or optical perturbations can affect the parameters
associated with the laser’s dynamics, such as threshold gain, damping, spectral linewidth,
and mode selectivity. The external perturbations can often produce undesired instabilities
in the laser, but they can be well controlled to produce desirable laser properties
including improvement in the modulation characteristics and spectral stability.
The objective of this work is to implement common external control techniques to
investigate their impact on the overall behavior of nanostructure lasers with a focus on
improvement in direct modulation performance and spectral characteristics. The external
control techniques studied in this dissertation include optical injection-locking, optical
feedback, and asymmetric bias control. These techniques were previously studied
thoroughly in bulk and QW lasers, but very little is known about the impact of external
control on performance of nanostructure lasers including modulation characteristics and
wavelength stability.
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5
The work presented in this dissertation initially focuses on modeling the direct
modulation behavior of injection-locked QD and QDash nanostructure lasers under
strong optical injection and stable locking conditions. The existing modulation response
model is reformulated using small-signal analysis of the coupled rate equations of the
master and slave lasers. As a result, a novel modulation response function is developed
that allows one to extract the key operating parameters of the system directly from
measured microwave response data. The significance of this modeling approach is that it
allows all the external control parameters to be extracted in the frequency domain where
they can be easily compared in order to further aid in optimizing the modulation
performance of the system. The model presented incorporates the impact of nonlinear
gain, which is known to be significant in QD and QDash lasers, along with the field
enhancement factor relating the deviation of the steady-state slave field amplitude from
its free-running value at high injection strengths. This is one of the major differences in
modeling the modulation response of an injection-locked QD and QDash laser. Using the
simulation results, the impact of intrinsic slave parameters, including the free-running
relaxation oscillation, linewidth enhancement factor, and damping rate, on the injection-
locked modulation transfer function are studied.
Unique carrier dynamics in nanostructure lasers allows for the linewidth
enhancement factor to fluctuate within a large range in these devices [23]. Previous
studies found that the linewidth enhancement factor in nanostructure QD and QDash
lasers strongly depends on the photon density due to the gain compression enhancement
with carrier density in theses devices [22]. This work aims to investigate the impact of
strong optical injection on the slave linewidth enhancement factor found in QD and
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6
QDash lasers. The theoretical predictions and experimental findings presented in this
dissertation show that the free-running linewidth enhancement factor in QDash lasers can
be manipulated due to the significant threshold gain shift under strong optical injection.
This novel finding along with the enhanced bandwidth advantages offered in the direct
modulation of injection-locked nanostructure lasers promises a path to realizing a new
generation of compact, chirp-free, and ultrafast (>100 Gb/s) optical sources for data
transmission.
The second part of the dissertation focuses on generating dual-color emission in a
single laser diode by using the external control techniques of optical feedback and
asymmetric pumping. The device used for the dual-color demonstration was a two-
section QD Distributed Feedback (DFB) laser diode with the Bragg wavelength only
coupled to the excited state. Specifically, this part of the dissertation describes in detail
how the dual-color operation is realized through simultaneous ground state emission that
is uncoupled to the Bragg grating due to significant inhomogeneous broadening common
in QD active region. In previous works, high performance dual-color optical sources have
been intensively studied for terahertz (THz) signal generation using photomixing
techniques [24]. Considering that the existing THz generation techniques are mostly
reliant on bulk optics, this work was motivated by the need for compact and low-cost
THz sources using externally controlled semiconductor lasers. In this study the technical
design and fabrication of the two-section QD DFB laser diode is described. With the
DFB device biased well above its threshold, it is shown that either applying external
optical feedback or asymmetric pumping generates two single-mode emission peaks in
the optical spectra–one line from the ground state and the other from the excited state.
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7
The origin of this dual-color mechanism is shown to be associated with manipulating the
QD carrier dynamics through controlled external perturbations provided by optical
feedback and/or asymmetric pumping. The technical design and external control
approaches described in this work along with current on-chip photomixing capabilities
have potential in engineering a compact and low-cost CW THz source for future
applications.
The following sections provide general overviews and applications of the external
control techniques studied in this dissertation.
1.2.1 Optical Injection-Locking
Overview of the Injection-Locking Technique
Optical injection-locking of semiconductor lasers involves two laser diodes often
referred to as the master and slave lasers as shown in Figure (1.1). There are two
configurations of injection-locking systems. One choice is to inject the master light into
one of the slave’s facets and collect the light from the other facet. Usually an optical
isolator is placed between the master and slave lasers to prevent reflections back to the
master. In a simpler configuration, an optical circulator can be used such that the light
from the master is injected from one slave facet and the output is collected from the same
facet. The latter method is easier since it only requires one fiber coupled to the slave
laser. The master laser is usually a single-mode, low linewidth, and high power tunable
laser. Several commercially available laser sources can be used as the master, including
external cavity lasers with wide wavelength tunability and highly linear, high-power
butterfly-packaged DFB lasers capable of wide temperature tuning of the wavelength.
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8
Slave laserTunable
Master laser
1550nm1550nm1550nm1550nm
Slave laserTunable
Master laser
1550nm1550nm1550nm1550nm
Figure (1.1) Schematic diagram of an optically injection-locked laser system. The red arrows represent the light emission from each laser. This is the less common configuration for the coupled system. Both mirrors of the slave laser in this arrangement would have to be partially reflecting. The more advance version, which uses an optical circulator between the master and slave lasers, is shown in chapter 2, Figure (2.3).
Figure (1.2) Optical spectra of an injection-locked Fabry-Perot Laser.
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9
When the locking conditions are satisfied, the frequency of the slave is locked to
that of the master with a constant phase offset. Figure (1.2) shows measured optical
spectra of a Fabry-Perot (FP) slave laser with and without optical injection. As shown in
this figure, optical injection suppresses all FP modes, resulting in single mode operation.
There are two primary injection-locking parameters; frequency detuning ∆f, and
external injection ratio, Rext. Frequency detuning is defined as the frequency offset
between master and slave laser. The external injection ratio is the master to slave power
ratio at the slave emitting facet. As the frequency of the master is changed, the slave
mode will follow that frequency until the system becomes unlocked. The locking range
depends on several parameters including the external power ratio, coupling coefficient
and linewidth enhancement factor of the slave laser.
A Brief History of Optical Injection-Locking
The concept of frequency locking between two coupled oscillators has attracted
many researchers for centuries. The idea goes back to the 17th century, when Huygens
discovered the synchronization phenomenon between two clock pendulums mounted on
the same wall [25]. It was not until the early 1980s that the fundamental theory and the
idea of frequency synchronization between two semiconductor lasers was first studied by
Lang [26] and the benefits of this technique on actual devices was verified [27]. Most of
the early research was focused on weak optical injection which typically yields a small
locking range. It was found that this regime leads to unstable locking regime that exhibits
resonant oscillation sidebands and chaotic behaviors [26], [28]-[32]. By the mid 1980s,
further developments on the injection-locking technique explored the advantage of the
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10
stable injection-locking in semiconductor lasers. It was found that using optical injection-
locking as an external control technique in CW lasers can significantly reduce the laser’s
spectral linewidth and noise [33], [34]. These findings were considered as the most
important benefits of optical injection-locking. In the meantime, with the advancement in
laser designs and efficient material systems, the advantages of injection-locking on
directly modulated lasers for applications such as coherent optical communications was
realized [35], [36]. The improvements to the modulated free-running slave laser due to
injection-locking include an enhancement in the relaxation oscillation, a reduction in
laser relative intensity-to-noise (RIN), a reduction in nonlinear distortion, and most
importantly reduction in linewidth and chirp [28], [37]-[46].
Typically, the modulation bandwidth of free-running semiconductor lasers is limited
as a result of the resonance frequency and damping rate set by the K-factor [47]. It has
been shown that the resonance frequency enhancement with optical injection-locking can
result in improving the overall bandwidth [48]-[52]. Figure (1.3) shows the experimental
results of an optically injection-locked laser demonstrating this improvement. The
characteristics of the modulation response curves in Figure (1.3) are shown to vary as a
function of the frequency detuning for constant injection strength. It is important to note
that improvement of the modulation bandwidth does not always include the enhancement
of the 3-dB bandwidth with resonance frequency. This is due to the occurrence of the pre-
resonance dip observed at specific detuning conditions as shown in Figure (1.3).
Depending on the detuning condition, enhanced modulation response of an injection
locked laser can fall into broadband or narrowband regimes of operation as illustrated in
Figure (1.3). Using optical injection-locking, a narrowband response with >80 GHz
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11
Figure (1.3) Measured modulation responses of an injection-locked FP laser, for various detuning conditions, indicating free-running, broadband and narrowband responses.
intrinsic bandwidth has been reported in a QW vertical cavity surface emitting laser
(VCSEL) operating at 1510 nm [53]. Hwang et. al have demonstrated a two-fold
improvement in the broadband response compared to the free-running case on a DFB
laser at 1310 nm under strong optical injection [52].
Another important improvement in optically injection-locked systems over the free-
running lasers is the reduction in linewidth and chirp. One important figure of merit in
directly modulated free-running lasers is the bit rate-length (BL) product. Limited
transmission distances in optical fiber links are mainly due to the linewidth broadening
caused by frequency chirp. Reduced chirp by injection-locking decreases the linewidth
broadening thereby reducing the pulse broadening caused by dispersive fibers. This
important feature of injection-locking was reported to create low chirp, allowing for long-
haul transmission with improved BL product [35].
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12
Applications of Optically Injection-Locked Lasers
Recently, the external control technique of free-running lasers through optical
injection-locking has been implemented in several state-of-the-art applications including
millimeter-wave generation [54], all-optical signal processing [55], radio over fiber [56],
and cable access TV (CATV) [57].
Goldberg et al., first demonstrated millimeter-wave generation by the sideband
injection-locking technique with sub-Herz RF linewidths [58], [59]. In this technique,
two slave lasers were coherently coupled by locking them to the adjacent sidebands of the
master laser. Using the millimeter carrier signal generated by sideband injection-locking,
a 64 GHz carrier for data transmission at a rate of 155 Mb/s over ~13 km single mode
fiber was demonstrated [60], [61]. With the new advancements in the field of millimeter
wave generation using optically injection-locked sources, more compact designs along
with tunability features have been developed that use only one modulated slave and a
master laser as demonstrated in an injection-locked two-section DFB device [62], [63].
Pulse broadening due to fiber dispersion is one of the major limitations in long-haul
digital communication systems. To prevent the in-line data loss and increased bit error
rate, several electro-optical repeaters are typically required to regenerate and reshape the
optical signal along the fiber link. The main drawback of using these repeaters is that
they increase the cost and complexity of the system and they also introduce additional
speed limitations as a result of electrical to optical and optical to electrical conversions.
Fortunately, all-optical signal regeneration and pulse reshaping have been realized
through a side-mode injection-locked DFB laser and a double injection-locked FP laser
[64], [65]. This technique is based on switching the locking stability as a function of
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13
injection strength. At a fixed frequency detuning, the locking threshold condition of the
slave laser defines the lock or unlock states in the coupled system. In this method, the
master laser is modulated with the digital signal and then it is weakly injection-locked
into a side-mode of a DFB slave laser. At a digital “1” state, the injected power is enough
to lock the master to slave and the slave output contains the master frequency
information. At a digital “0” state, small injected power leads to the unlocked situation,
and the output will be that of the slave. A bandpass filter is typically used to only transmit
the master frequency at the output. Therefore, the abrupt threshold of the locking and
unlocking processes is used to reshape the distorted signals, resulting in a frequency
modulated signal with reduced noise.
Improved modulation characteristics of optical injection-locked lasers including the
enhanced bandwidth and reduced chirp have been implemented in radio-over-fiber and
CATV applications [56], [57]. In a recent study, the external optical injection technique
has been employed in a hybrid radio-over-fiber system to improve the bit error rate
performance [56]. Using the resonance frequency enhancement in an injection-locked
DFB laser modulated with 125 Mb/s digital signals, narrowband transmission at the sharp
resonance peak is demonstrated. A similar approach has been applied to a CATV
transmission experiment using a directly modulated DFB laser under strong optical
injection. The modulated signals were up-converted to the enhanced resonance frequency
at 18.5 GHz and transmission parameters were compared to the baseband free-running
transmission values. Comparing the transmission experiments under strong optical
injection with the free-running results showed a 3-dB improvement in composite second
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14
order (CSO) and composite triple beat (CTB) performances over a 100 km single mode
fiber transmission [57].
1.2.2 External Control through Optical Feedback
Overview of External Optical feedback
Semiconductor lasers subjected to external optical feedback are known to exhibit
very interesting nonlinear dynamics, which either lead to instabilities and chaotic
behaviors at the laser output or result in practically useful impacts that can improve the
device’s intrinsic characteristics. Commercial application of semiconductor lasers in
optical fiber links was the first practical motivation for studying the behavior of
semiconductor lasers subject to optical feedback. Even a small back reflection from the
fiber pigtail tip or optical fiber connectors into the diode laser module was shown to
degrade the modulation characteristics and increase the intensity noise [66], [67]. To
prevent these undesired effects, the laser diode transmitter modules are usually
accommodated with an optical isolator which rejects any back reflection but increases the
cost of using laser diodes in optical fiber links.
In 1980, Lang and Kobayashi reported on some aspects of the statics and dynamics
of the semiconductor laser exposed to the external optical feedback from a distant
reflector [68]. In that study, the authors brought to light the intrinsic characteristics of
laser gain media including the broad gain spectrum, temperature dependence of the
material refractive index, and the carrier density dependence of the refractive index, each
exhibiting complex behavior under external feedback conditions. They also reported on
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15
experimental observation of bistability and hysteresis in the laser output light output as a
function of injected current characteristics.
Since then, understanding the effects of optical feedback on both dynamical and
spectral features of semiconductor lasers has been pursued extensively [69]-[71]. Early
studies on the spectral characteristics showed that the laser linewidth could be either
narrowed [70] or broadened [69] under the influence of optical feedback. These
behaviors were initially understood through the spectral response sensitivity to the phase
of the reflected light. Goldberg et. al have demonstrated that by changing the feedback
parameters somewhat, multi-stability can be observed as the system performs “mode-
hops,” where a laser diode operates on a single external cavity mode for some time, but
then suddenly switches to another [72]. Another form of bistability, referred to as “low-
frequency fluctuations” was studied by Mørk et. al [73]. This form of instability is
observed when the laser is pumped close to the threshold and is subject to moderate
feedback levels. At this condition, laser output shows sudden drops followed by a gradual
build-up. The noise properties of semiconductor lasers subject to optical feedback have
attracted considerable theoretical and practical interest [74], [75]. More generally, much
effort has been devoted to modeling the impact of optical feedback on the dynamical
behavior of semiconductor lasers [76], [77].
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16
Figure (1.4) Regimes of optical feedback for a DFB laser indicating the feedback power ratio at which the transition between regimes occur as a function of external round-trip time [79].
Regimes of Optical Feedback
It has been experimentally and theoretically shown that the effects of external optical
feedback on laser dynamics or spectral properties can be different depending on several
factors including the laser bias condition, strength and phase of optical feedback, and the
distance between the external reflector and the laser cavity [78]. It was based on these
observations that Tkach and Chaprylyvy first experimentally introduced “the regimes of
optical feedback” in semiconductor lasers [79]. In that study, five identifying operating
regimes of feedback were characterized by reference to either the dynamical or spectral
properties of the laser and conventionally labeled as regimes I to V as shown in Figure
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17
(1.4). In regime I, under weak feedback levels, the laser linewidth can be either narrowed
or broadened depending upon the phase of the optical feedback. Regime II is
characterized by the appearance of longitudinal mode hopping. In regime III the laser
becomes stable and locks to the mode with minimum single-mode linewidth. In regime
IV, with increasing feedback level, the linewidth of the laser dramatically broadens. This
phenomenon is referred to as “coherence collapse.” Further increasing the feedback
strength into regime V, the laser enters a stable external cavity mode operation. There
have been extensive studies made of the five feedback regimes of semiconductor laser
operation [74]. In particular, considerable effort has been given to determining the nature
of the laser dynamics in the coherence collapse regime, or regime IV, as it was first
reported by Lenstra et al. [80].
Advantages of Controlled Optical Feedback
As mentioned before, optical feedback has been shown to produce deleterious effects
on semiconductor lasers including significant linewidth broadening and mode-hopping.
However, based on the observed impacts on the dynamical and spectral properties,
controlled external feedback is predicted to have much potential in stabilizing and
improving laser performance. Recent advancements in modeling the nonlinear dynamics
of semiconductor lasers subject to optical feedback have provided a path to understand
the associated instabilities and develop methods for controlling the useful underlying
dynamics for practical applications [76], [80].
Advantages of controlled feedback have been realized in earlier studies where it was
showed that adjusting the feedback level and phase matching can result in a stable
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18
operation with considerable spectral linewidth narrowing [81]. Coherent feedback control
has also been found to be useful in enhancing the relaxation oscillations and reducing the
signal distortion in the modulated laser output [82]. These effects are very important for
the laser especially when it is implemented in coherent communication systems.
In another control method of optical feedback, a frequency filter is typically used to
access a desired dynamical behavior in a specific region by restricting the phase space
that is available to the feedback laser system [83]. Lately, the idea of controlling the
nonlinear dynamical behavior in semiconductor lasers has been developed to utilize
chaotic dynamics in applications such as chaos synchronization for secure
communication systems [84]. The filtered optical feedback technique has become an
interesting topic [85], since it can control the laser dynamics through two external
parameters: the spectral width of the filter and frequency detuning of the free-running
laser. The frequency filter method was shown to provide a mechanism for controlling the
impact of relaxation oscillations on the dynamical response of the laser as well as
permitting an external control over the nonlinearities of the device [83]. Using this
approach, tunable and pure frequency oscillations in the solitary laser can be generated by
detuning the frequency of the optical feedback through a Fabry-Perot resonator [86].
The external control through delayed optical feedback has recently found its way
into the field of semiconductor passively mode-locked lasers. Recent studies have been
both experimentally and theoretically shown that controlled external optical feedback is a
simple and efficient method to improve the RF linewidth and timing stability via
reducing the RF phase-noise in passively mode-locked lasers [87], [88].
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Applications of Semiconductor Lasers with External Optical Feedback
Applications of semiconductor lasers with controlled external optical feedback are
driving rapid developments in theoretical and experimental research. The very broad
gain-bandwidth of semiconductor lasers combined with frequency-filtered, strong optical
feedback create the tunable, single frequency laser systems utilized in
telecommunications, environmental sensing, measurement and control [89]. Those with
weak to moderate optical feedback levels lead to the chaotic semiconductor lasers which
can be implemented in secure communication systems [90].
1.2.3 External Control through a Monolithic Multi-Section Design
In the early1960s, after the invention of DH semiconductor lasers, stabilizing the
single-longitudinal mode operation in semiconductor lasers was one of the most
important challenges in developing high bit rate and single mode fiber transmission
systems [91]. For this reason, several optical integration approaches have been proposed
including, but not limited to: external cavity lasers with spherical mirrors, distributed
Bragg reflector (DBR) lasers, cleaved-coupled cavity (C3) lasers, and monolithic two-
section lasers. The idea of developing coupled-cavity lasers [92] was initially based on a
simple method to improve the single-mode stability under high modulation frequencies
and temperature fluctuations in semiconductor lasers [93]. A similar idea was later
proposed to implement a coupled-cavity design for controlling the laser output, such as
wavelength tuning, by separately pumping the individual cavity [94].
The 1980s saw rapid development of new designs and theoretical studies for
coupled-cavity lasers [95]-[97]. With further developments in device fabrication
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techniques, monolithic multi-section lasers, a cousin of the C3 laser, have become readily
available for new research topics and developing applications including direct
modulation, passive mode-locking, and wavelength tuning [98]-[101]. Samples of the
interesting applications for multi-section laser devices are presented below.
Optical Gain-Lever
The optical gain-lever was first realized by K. J. Vahala, et al. in 1989 by
demonstrating the enhancement in modulation efficiency produced by either optical or
electrical modulation of laser cavity [102]. Before that, the idea of producing parasitic-
free modulation in semiconductor lasers was developed using a technique called “active
layer photo-mixing” [103]. In this method, the light produced by two single mode laser
sources was mixed and optically pumped the active layer of another laser diode,
producing a carrier density modulation. In 1989, N. Moore and K. Y. Lau [104]
suggested that a two-segment configuration in a laser diode could be used to produce a
net gain in the conventional carrier modulation of semiconductor lasers. Specifically, Lau
studied the optical gain-lever effect to enhance the efficiency of direct intensity
modulation and optical frequency modulation of a two-section QW laser [105].
Figure (1.5) shows the schematic view of the two-section gain-lever configuration
with a typical gain versus carrier density characteristic of a semiconductor laser. In the
two-section laser, each section is biased at different pump levels, where the net bias
results in a lasing condition. As shown in Figure (1.5), under asymmetric current
injection, the short section (a) referred to as the modulation section, is DC biased at a
lower gain level than the long section (b) termed the gain section. This Bias scheme
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Figure (1.5) Schematic diagram of a two-section gain-lever semiconductor laser with the evolution of gain versus carrier density [106].
provides a high differential gain under small-signal RF modulation. The gain section is
only DC biased and provides most of the optical amplification, but at a relatively smaller
differential gain. Since the total gain is clamped above the threshold and due to the non-
linear dependence of gain with carrier density, any small change in carrier density in the
modulation section produces a much larger variation in carrier density in the gain section
and consequently in the total photon density. In such a case an RF optical gain will result
when the differential gain in the modulation section, G’a, is greater than the differential
gain in the gain section, G’b.
Recently, much research work based on the gain-lever effect has been conducted to
improve modulation characteristics of QD nanostructure lasers. 8 dB and 20 dB intensity
modulation efficiency enhancements were demonstrated using p-doped and un-doped QD
lasers respectively [106], [107]. It was also theoretically and experimentally
demonstrated that under an extreme asymmetric bias configuration, a QD laser
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employing the gain-lever can exhibit a two-fold enhancement in the 3-dB bandwidth
compared to a regular single-section QD laser [108].
Multi-Section DFB/DBR Lasers
Tuning the laser frequency is very important for a variety of applications in coherent
optical communication, such as wavelength division multiplexing (WDM), heterodyne
detection systems, frequency modulation, and optical switching in local area networks
[98], [100], [109]. Depending on the application, the tuning mode, range, and speed
requirements are different. Usually for WDM applications, large and continuous
frequency tuning is desired (>1 THz), while the frequency modulation requires small but
fast frequency shifts ( 40 nm), and
narrow linewidth performance [113].
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In addition to frequency tuning features, advanced DBR lasers have been widely
studied for various applications such as optical communication systems. In recent studies,
the optical injection locking technique has been combined with gain-lever modulation in
multi-section DBR lasers to further improve the RF performance, including the
modulation efficiency enhancement, bandwidth improvement and nonlinear distortion
reduction. Using this technique, a 10-dB enhancement in the intensity modulation
efficiency, a 3x improvement in the modulation bandwidth, and a 15-dB suppression of
the third-order inter-modulation distortion has been reported in a multi-section QW DBR
laser [114].
1.3 Organization of Dissertation
This work studies the manipulation of intrinsic characteristics of nanostructure QD
and QDash lasers using external control techniques of optical injection-locking, optical
feedback, and asymmetric bias configuration.
Chapter 2 studies the impact of optical injection-locking on the direct modulation
characteristics of QD and QDash nanostructure lasers. This chapter recasts a predictive
response model to investigate the modulation characteristics of QD and QDash lasers
under stable injection-locking conditions. The presented response function accounts for
the unique carrier dynamics in these lasers by implicitly incorporating the nonlinear gain
compression through known free-running parameters. Using this model, the key
operating parameters of injection-locked QDash and QD lasers were extracted directly
from measured data in the microwave domain and the results are compared for each laser
structure. Chapter 2 gives a detailed characterization of the slave QDash and QD devices
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under investigation and highlights the key free-running parameters that make these types
of active regions suitable for implementation in an injection-locked laser system. The
validity of the response model then is examined based on the extracted value’s
correlation with theoretical predictions. It is also shown that comprehensive analysis of
the extracted parameters can be further used in optimizing the overall modulation
performance of the coupled system.
In chapter 3, the impact of optical injection-locking on the linewidth enhancement
factor of a QDash slave laser is investigated. In this chapter, manipulation of the
linewidth enhancement factor of an injection-locked QDash laser under zero detuning
and strong optical injection is studied. The experimental findings are validated by
comparing the extracted linewidth enhancement values from the measured microwave
response to the directly measured results.
Chapter 4 describes methods of external control in the generation and stabilization
of dual-mode emission in a single frequency QD laser diode. In this chapter, a dual-
mode emission mechanism is realized for the first time by asymmetrically pumping a
two-section QD DFB laser operating in the excited state mode. The detailed design and
fabrication of the two-section QD DFB laser diode is presented. This chapter also
demonstrates how combining the unique QD carrier dynamics along with excited state
coupled mode operation allows for the manipulation of QD states through external optical
feedback stabilization.
Lastly, chapter 5 gives a summary of the work and highlights proposed future
research related to the impact of external control techniques on overall characteristics and
performance of QD and QDash semiconductor lasers.
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1.4 Chapter 1 References
[1] R. N. Hall, G. E. Fenner, J. D. Kingsley, T. J. Soltys, and R. O. Carlson,
“Coherent Light emission from GaAs junctions,” Physics Review Letters, Vol. 9,
No. (9), pp. 366-368, (1962).
[2] Zh. I. Alferov, V. M. Andreev, E. L. Portnoi, M. K. Trukan, “AlAs-GaAs
heterojunction injection lasers with a low room-temperature threshold,” Soviet
Physics -Semiconductors, Vol. 3, pp. 1107-1110, (1970).
[3] Zh. I. Alferov, V. M. Andreev, D. Z. Garbuzov, Yu. V. Zhilyaev, E. P. Morozov,
E. L. Portnoi, and V. G. Trofim, “Effect of heterostructure parameters on the laser
threshold current and the realization of continuous generation at room
temperature,” Soviet Physics -Semiconductors, Vol. 4, pp. 1573-1575, (1970).
[4] I. Hayashi, M. B. Panish, P. W. Foy and S. Sumski, “Junction lasers which
operate continuously at room temperature,” Applied Physics Letters, Vol. 17, No.
(3), pp. 109-111, (1970).
[5] R. D. Dupuis, and P. D. Dapkus, “Very low threshold Ga1-xAl xAs-GaAs double-
heterostructure lasers grown by metalorganic chemical vapor deposition,” Applied
Physics Letters, Vol. 32, No. (8), pp. 473-475, (1978).
[6] C. Gmachl, F. Capasso, D. L. Sivco, and A. Y. Cho, “Recent progress in quantum
cascade lasers and applications,” Reports On Progress In Physics, Vol. 64, No.
(11), pp. 1533-1601, (2001).
[7] Z. I. Alferov, V. M. Andreev, V. I. Korol’kov, E. L. Portnoi,